1. Scope of the Invention
The present invention relates to Al--Mg based alloy sheets with good press formability, more specifically, excellent stretchability, superb deep drawability and high forming limits in the uniaxial tension to plane strain tension region. These Al--Mg based alloy sheets are suitable for automotive applications and the like.
2. Description of the Prior Art
From the consideration of recent concerns for the global environment, social demands toward reducing the weight of automobiles to reduce fuel consumption have escalated. To satisfy such demands, the application of aluminium materials to replace steel sheet for automotive parts has been investigated.
While aluminium alloy sheets have strengths at almost the same level as those of conventional steel sheets, they are generally poorer in press formability such as deep drawability and stretchability. As a result, the improvement of aluminium alloy sheets in terms of press formability has been strongly demanded by automotive manufacturers.
Conventionally, aluminum alloy materials of the Al--Mg series, such as JIS 5052 alloy or JIS 5182 alloy, and the Al--Mg based alloy material disclosed in Japanese Patent Laid-open No. Sho 52-141409, have been used for applications requiring excellent press formability. The present inventors have made investigations and conducted research, development, and merchandising, which led to the development of KS5030 alloy and KS5032 alloy (both under the trade names of Kobe Steel, Co.; the contents thereof are disclosed in Japanese Patent Laid-open Nos. Sho 60-125346, Sho 63-89649, Hei 2-269937 and Hei 3315486). These alloys are characterized as having high strength and high ductility due to the addition of a relatively high amount of Mg. These alloys have enhanced mechanical properties after painting and curing and better stress corrosion cracking resistance through the addition of Cu at about 0.5 wt. %. Additionally, these alloys have optimum grain sizes through the addition of Mn and Cr. These aluminium alloy sheets are used to make automotive parts and the like.
However, the formability of these aluminium based alloy sheets is not satisfactory for many applications, so automobile manufacturers have demanded further improvements in formability. One of the reasons why the formability is insufficient is because aluminium's plastic anisotropy cannot be controlled well. No attention has been paid toward crystallographic texture control as a means to influence the plastic anisotropy of JIS alloys such as JIS 5182 or Al--Mg based alloys disclosed in Japanese Patent Laid-open Nos. Sho 52-141409, Sho 60-125346, Sho 63-89649, Hei 2-269937 and Hei 3-315486 wherein, only the chemical compositions of these alloys are specified. Hence, the formability is insufficient.
It has traditionally been known that crystallographic texture is an important microstructural feature in the control of the formability. It is known that the deep drawability of cold-rolled steel sheets can be improved by promoting a {111} texture, i.e., the normals of {111} crystallographic planes are nearly parallel to the sheet normal direction. It has been proposed in recent years that the formability of aluminium alloys can also be improved by controlling the crystallographic texture. For example, Japanese Patent Laid-open No. Hei 5-295476 discloses an Al--Mg based alloy sheet, wherein the volume fraction of the {110} texture (grains with {110} crystallographic planes nearly parallel to the sheet plane) is 10% or more, the ratio of the volume fraction of the {110} texture to the volume fraction of the {112} texture is 1.5 or more, and the grain size is in the range of 35 to 80 .mu.m. However, the crystallographic texture disclosed therein is not optimum for deep drawing.
The Al--Mg alloy disclosed in Japanese Patent Laid-open No. Hei 8325663 was developed with attention focused on stretchability, while no consideration was paid to the grain structure which largely controls the drawability. Therefore, the alloy cannot achieve satisfactory press formability.
In an academic paper, by using computer simulations based on the theory of plastic deformation, P. Ratchev et al. made an assumption about the relationship between the crystallographic texture of Al--Mg alloy sheet and formability. He reported that a crystallographic texture with a strong Cube orientation might result in greater anisotropy, leading to the reduction of the formability (Texture and Microstructures, Vol.22, p.219, 1994).